Add alerthub documentation

docs/model/sources/AlertHub/README.md

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+# AlertHub
+
+The aim of IFRC Alert Hub is to provide timely and effective emergency alerts to communities worldwide, empowering them to protect their lives and livelihoods.
+
+## Collection: `alerthub-events`
+
+A STAC collection hold all the AlertHub events. An example of the AlertHub collection is [here](https://github.com/IFRCGo/monty-stac-extension/blob/main/examples/alerthub-events/alerthub-events.json)
+
+- Name: AlertHub
+- Code: `AlertHub`
+- Source organisation: IFRC
+- Source code: alerthub
+- Source type: IFRC
+- Source organization email: im@ifrc.org
+- Source URL: <alerthub.ifrc.org>
+- Source Data license: MIT License
+- Source for: event, hazard
+
+### Data
+
+The data from Alerthub can be accessed using the GraphQL queries. The data is currently fetched using the query `Alerts` from the following [data source](https://alerthub-api.ifrc.org/graphql/).
+
+### Event Item
+
+An AlertHub event and episode will **ALWAYS** produce and [event STAC item](https://github.com/IFRCGo/monty-stac-extension#event). Here is a table with the fields that are mapped from the AlertHub event to the STAC event.
+
+| STAC field | AlertHub | Field Description |
+|------------|----------|--------------------|
+| [id](https://github.com/radiantearth/stac-spec/blob/master/item-spec/item-spec.md#id) | alert.id | Unique identifier for the event |
+| [geometry](https://github.com/radiantearth/stac-spec/blob/master/item-spec/item-spec.md#geometry) | alert.info.areas | Geometry of the event |
+| [bbox](https://github.com/radiantearth/stac-spec/blob/master/item-spec/item-spec.md#bbox) | From geometry | Bbox is constructed using geometry data using bounds |
+| [collection](https://github.com/radiantearth/stac-spec/blob/master/item-spec/item-spec.md#collection) | `alerthub-events` | The collection for AlertHub events |
+| [title](https://github.com/radiantearth/stac-spec/blob/master/commons/common-metadata.md#basics) | alert.headline | Short description of the event |
+| [description](https://github.com/radiantearth/stac-spec/blob/master/commons/common-metadata.md#basics) | alert.description | Description of the event |
+| [datetime](https://github.com/radiantearth/stac-spec/blob/master/commons/common-metadata.md#date-and-time) | alert.effective | Date and time of the event converted in UTC ISO 8601 format |
+| [start_datetime](https://github.com/radiantearth/stac-spec/blob/master/commons/common-metadata.md#date-and-time-range) | alert.effective | Date and time of the event converted in UTC ISO 8601 format |
+| [end_datetime](https://github.com/radiantearth/stac-spec/blob/master/commons/common-metadata.md#date-and-time-range) | alert.expires | End date of the event converted in UTC ISO 8601 format |
+| [monty:episode_number](https://ifrcgo.org/monty-stac-extension/v1.0.0/schema.json#monty:episode_number) | 1 | Set to 1 as there is no proper episodes handling mechanism |
+| [monty:country_codes](https://github.com/IFRCGo/monty-stac-extension#montycountry_codes)\[0] | alert.country.iso3 | ISO3 of the country |
+| [monty:hazard_codes](https://github.com/IFRCGo/monty-stac-extension#montyhazard_codes) | alert.category, alert.event | List of hazard codes |
+| [`via` link](https://github.com/radiantearth/stac-spec/blob/master/commons/assets.md) in \[links] | alert.url | Link to the GDACS event details page |
+| [monty:corr_id](https://ifrcgo.org/monty-stac-extension/v1.1.1/schema.json#monty:corr_id)                                   | Generated                                          | Generated following the [event correlation](../../correlation_identifier.md) convention |
+| [monty:guid](https://ifrcgo.org/monty-stac-extension/v1.1.1/schema.json#monty:guid) | Generated | Generated following the [guid string](../../global_identififer.md) convention |
+
+
+### Hazard Item
+
+An AlertHub will **ALWAYS** produce one [**hazard STAC item**] (https://github.com/IFRCGo/monty-stac-extension#hazard).
+
+Here is a table with the STAC fields that are mapped from the AlertHub event to STAC hazard.
+
+| STAC field | AlertHub | Field Description |
+|------------|----------|--------------------|
+| [id](https://github.com/radiantearth/stac-spec/blob/master/item-spec/item-spec.md#id) | alert.id | Unique identifier for the event |
+| [geometry](https://github.com/radiantearth/stac-spec/blob/master/item-spec/item-spec.md#geometry) | alert.info.areas | Geometry of the event |
+| [bbox](https://github.com/radiantearth/stac-spec/blob/master/item-spec/item-spec.md#bbox) | From geometry | Bbox is constructed using geometry data using bounds |
+| [collection](https://github.com/radiantearth/stac-spec/blob/master/item-spec/item-spec.md#collection) | `alerthub-hazards` | The collection for AlertHub events |
+| [title](https://github.com/radiantearth/stac-spec/blob/master/commons/common-metadata.md#basics) | alert.headline | Short description of the event |
+| [description](https://github.com/radiantearth/stac-spec/blob/master/commons/common-metadata.md#basics) | alert.description | Description of the event |
+| [datetime](https://github.com/radiantearth/stac-spec/blob/master/commons/common-metadata.md#date-and-time) | alert.effective | Date and time of the event converted in UTC ISO 8601 format |
+| [start_datetime](https://github.com/radiantearth/stac-spec/blob/master/commons/common-metadata.md#date-and-time-range) | alert.effective | Date and time of the event converted in UTC ISO 8601 format |
+| [end_datetime](https://github.com/radiantearth/stac-spec/blob/master/commons/common-metadata.md#date-and-time-range) | alert.expires | End date of the event converted in UTC ISO 8601 format |
+| [monty:episode_number](https://ifrcgo.org/monty-stac-extension/v1.0.0/schema.json#monty:episode_number) | 1 | Set to 1 as there is no proper episodes handling mechanism |
+| [monty:country_codes](https://github.com/IFRCGo/monty-stac-extension#montycountry_codes)\[0] | alert.country.iso3 | ISO3 of the country |
+| [monty:hazard_codes](https://github.com/IFRCGo/monty-stac-extension#montyhazard_codes) | alert.category, alert.event | List of hazard codes |
+| [`via` link](https://github.com/radiantearth/stac-spec/blob/master/commons/assets.md) in \[links] | alert.url | Link to the GDACS event details page |
+| [monty:corr_id](https://ifrcgo.org/monty-stac-extension/v1.1.1/schema.json#monty:corr_id)                                   | Generated                                          | Generated following the [event correlation](../../correlation_identifier.md) convention |
+| [monty:guid](https://ifrcgo.org/monty-stac-extension/v1.1.1/schema.json#monty:guid) | Generated | Generated following the [guid string](../../global_identififer.md) convention |
+| [monty:hazard_detail](https://github.com/IFRCGo/monty-stac-extension#montyhazard_detail) | alert.info.severity | Severity Info |
+
+
+#### Hazard Detail
+
+The [hazard_detail](https://github.com/IFRCGo/monty-stac-extension#montyhazard_detail) field is a JSON object that contains the detailed information about the hazard. The object is a mapping of the hazard codes to the detailed information. The detailed information is a JSON object with the following fields:
+
+| STAC field     | AlertHub field                  | Description                                               |
+| -------------- | ---------------------------- | --------------------------------------------------------- |
+| severity_value | Uses a severity mappings | Five enums are mapped to a numeric value |
+| severity_unit  | `alerthub`                      | AlertHub alert level |
+| estimate_type  | MontyEstimateType.PRIMARY | Use a fixed value |